1. Field of the Invention
The present invention relates generally to a optical illumination system of a projection system, and more particularly, to an optical device for homogenizing luminous energy and polarized light, and to an optical illumination system of a projector using the optical device whereby the volume of the optical illumination system of projector can be decreased.
2. Description of the Prior Art
Lately, as a substitution for a cathode-ray tube display which has the limits of screen size and bulky system, the supply of a projector, which is a kind of flat display having a relatively thin thickness but realizing capacity large sized screen, capable of magnifying and projecting a small picture on a large screen in size has rapidly increased.
Although a projector uses a display device, such as a cathode-ray tube, an LCD (Liquid Crystal Display), or a DMD (Digital Micro-mirror Device), capable of realizing a small picture in size, the LCD or the DMD is mainly used so as to satisfy thin-type trend.
The LCD realizes a picture by controlling the quantity of light transmission in accordance with the array of a liquid crystal that varies in response to change of electric signal applied from an external source. Then, the DMD realizes a picture by using two modes of the angle of reflection that are achieved by varying the angle of gradient of a micro-mirror between an angle of plus 10 degrees and an angle of minus 10 degrees in accordance with change of electric signal applied from an external source.
Such projector has developed on the basic axis of small size, lightweight, and high brightness.
Namely, so as to decrease in size and in weight, a projector develops from three flats type using three display devices into single flat type using one display device for realizing color.
The projector has also been improved so as to overcome the problem that clear screen could not be realized in bright circumstances. For example, a lamp as a light source has been improved into a lamp having a smaller emission in size, as shown in FIG. 1, light efficiency has improved by using Fly eye lens for homogenizing luminous energy, and polarized light transformer for transforming a light supplied from the light source into single linear polarized light.
FIG. 1 is a diagram showing the structure of the optical illumination system of a related art liquid crystal projector.
The structure and operation of the optical illumination system of a related art liquid crystal projector will now be described with reference to FIG. 1.
The optical illumination system includes a first and a second Fly eye lens (4, 6) arranged between a light source (2) and a display device (12) in series from the light source (2), a Polarizing Beam Splitter Array (hereinafter, a PBS array) (8), and a reverse Fourier transform lens (10).
The light source (2) includes an arc luminescence lamp (not shown) having small size luminescence and a parabolic reflection mirror (not shown). The arc luminescent white light from the light source (2) is reflected on the parabolic reflection mirror and travels to the first Fly eye lens (4) in a semi parallel light form.
The first and the second Fly eye lens (4, 6) include a plurality of sets of micro lens arranged in a matrix form.
An incident light from the light source (2) is divided through the first Fly eye lens (4) in terms of sets of lenses, and is incident over respective sets of lenses of the second Fly eye lens (6). The second Fly eye lens (6), which is a first focal length distant from the first Fly eye lens (4), transforms the incident light from the first Fly eye lens (4) into a parallel light and makes the parallel light travel to the PBS array (8). The second Fly eye lens (6) is also named Fourier transform lens.
By means of the first and the second Fly eye lens (4, 6), luminous energy incident over the display device (12) is homogenized.
The PBS array (8) divides an incident light from the second Fly eye lens (6) into linear polarized lights having an optical axis, that is, a vertical linear polarized light (hereinafter, P polarized light) and a horizontal linear polarized light (hereinafter, S polarized light).
FIG. 2 is a diagram showing a detailed structure of the PBS array (8).
The structure of the PBS array (8) and operations of dividing and homogenizing polarized light of the PBS array (8) will now be described with reference to FIG. 2.
The PBS array (8) includes a plurality of PBS prism bars (16) each of which has polarization division surface (18) obliquely installed between incident surface and emission surface for polarization dividing an incident light (14), and a plurality of a half wavelength plates (24) partially attached to the rear surface of the PBS array (8).
The incident light (14) incident on the PBS array (8) is divided as a result that a P polarized light (22) is transmitted and an S polarized light (20) is reflected on the polarization division surface (18) obliquely formed in the PBS prism bar.
In this way, the S polarized light (20) reflected on the first surface of the polarization division surface (18) is repeatedly reflected on the second surface of the polarization division surface (18) which is adjacent to the first surface, and is emitted like that through the emission surface of the PBS array (8). On the other hand, the P polarized light emitted through the polarization division surface (18) is transformed into S polarized light (20) and emitted by the half wavelength plates (24) partially attached to the rear surface of the PBS array (8).
Thus, incident lights are all transformed into a linear polarized light such as S polarized light by the PBS array (8), so that most light emitted from the light source (2) can be used for realizing a picture in the display device (12) such as liquid crystal panel using linear polarized light, and brightness can be improved consequently.
The PBS array (8) is attached to the rear surface of the second Fly eye lens (6) as one body.
The polarization divided and homogenized light transmitted through the PBS array (8) travels to the reverse Fourier transform lens (10).
Parallel incident S polarized light (20) from the PBS array (8) can be focused on the display device (20) in minimal angle of incidence by the reverse Fourier transform lens (10), so that the loss of light can be prevented.
In the optical illumination system having the above-described structure, the display quality of the display device (12) can be improved by using the Fly eye lens (4, 6) for homogenizing luminous energy and the PBS array (8) for homogenizing polarized light.
However, the Fly eye lens (4, 6) and the PBS array (8) have relatively bulky volume, so that the total volume of the optical system may be bulk. Thus, it is difficult to meet the small size and thin-type trend.